Transducers are commonly used in connection with bowed musical instruments in order to allow these instruments to be amplified, recorded or to allow them to control other instruments such as music synthesizers. When a bowed instrument is played by drawing the bow across a string, a friction noise is generated in the string at the point of contact between the string and the bow. This friction noise is transmitted through the string into the bridge of the instrument. In conventional acoustic instruments, this bow noise does not have much projection power and becomes less objectionable as the distance between the instrument and the listener is increased. The musician playing the conventional acoustic instrument may ignore this bow noise in the confidence that the listener does not perceive it to an objectionable degree.
A first problem exists when an electric instrument is being played and reproduced by an amplifier, that the player and the listener hear approximately the same sound. The listener is then subjected to a significantly greater amount of bow noise than he is normally subjected to when listening to an acoustic instrument. The disproportion in the bow noise level gives the sound of the instrument a "scratchy" character which is objectionable. This condition is worsened when the instrument has a bridge pickup. Since the bow noise enters the instrument through the bridge, there will consequently be a higher degree of bow noise with this type of pickup which otherwise transduces the desirable music vibrations in a very satisfactory manner.
A second problem exists when the pickup signal of an electric bowed instrument is analyzed to produce a fundamental frequency related signal, that the bow noise present in the pickup signal may interfere with the analysis and result in erroneous signals and correspondingly erroneous control commands. This is particularly objectionable when controlling a second instrument such as a music synthesizer.
A third problem exists when the direction of bowing is changed while playing an electric bowed instrument, that a thumping sound is generated as a result of the bow noise suddenly stopping and then starting again.
A fourth problem exists, when using frequency shifting devices such as transposers or frequency dividers of the gated-signal type for producing sub-octave tone signals, that the bow noise gets correspondingly lowered in frequency in each corresponding sub-octave voice thus generated, making the resulting complex tones sound rather artificial.
It is accordingly a broad object of the present invention to provide a tone signal filter which selectively removes the undesirable portion of friction noise present in the signal from a transducer monitoring the desirable string vibrations of a bowed musical instrument.
It is a more specific object of the present invention to effect this selective noise reduction without significantly altering the tonal balance of the tone signal from the transducer.
It is another object of the present invention to selectively remove the noises in the transducer signal resulting from changes in the direction of bowing of the instrument.
It is a further object of the present invention to provide a filter which passes the signal transients resulting from plucking the strings, in an independent manner and possibly in an independently adjustable manner.
It is a still further object of the present invention to provide a bow noise filter which may be used in conjunction with frequency shifting devices to produce realistic sub-octave voices having the tonal characteristics of the bowed instruments producing the corresponding frequency range.